$ perl -Mstrict -Mwarnings -E '               
say sub {                              
   my $v = shift;
   return $v if length $v < 4;
   my $o = 0;
   substr $v, $_, 0, q{,} for
      reverse map { $o -= 3 } 1 .. int( ( length( $v ) - 1 ) / 3 );
   return $v;
   }->( $_ ) for qw{
      1
      12
      123
      1234
      12345
      123456
      1234567
      12345678
      123456789
      1234567890
      };'
1
12
123
1,234
12,345
123,456
1,234,567
12,345,678
123,456,789
1,234,567,890
$